The present invention relates generally to a system for detecting the water content of a liquid mixture, and more particularly to a system which does so by enhancing the contribution of the electrical conductivity of the water to the overall conductivity of the mixture.
Generally, the quantitative determination of the water content of a multi-component liquid system can be done in a variety of ways, such as by titration with anhydrous reagents. However, the standard textbook procedures for such a determination generally are performed in a laboratory setting, under carefully controlled conditions. Therefore, they are not readily amenable to an on-line situation in which a rapid and accurate measurement is needed in a brief period of time, in less than ideal environments. A particularly critical application, and one which is gaining more attention due to the current world energy crisis, is the production of anhydrous (water-free) alcohol (usually ethanol) for combining with gasoline to produce gasohol. Since the presence of even the slightest amounts of water in the alcohol may have adverse effects on the performance of an engine, careful steps must be taken to ensure that all traces of water are eliminated from the final product before it is combined with the gasoline. Guaranteeing this water-free condition usually requires almost continuous monitoring of the output of the alcohol distillation column, and in such a situation the simplicity and fast response of the measurement scheme may be vital to achieving products within specification.
It has long been known that electrical conductivity can provide an accurate indication of the presence of water in mixed systems, and capacitance measurements have been used to detect water in jet fuels (kerosene). In fact, the use of conductivity measurements for detecting the presence of water is cited in an article entitled "Methods for the Determination of Water," by John Mitchell, Jr., included in Handbook of Analytical Chemistry, L. Meites, editor, McGraw-Hill, 1st edition, 1963. However, the cited article also comments that the presence of other conducting substances in variable amounts acts as an "interference," to hinder accurate measurement. Additionally, in the case where water is combined with alcohol, particularly ethanol or methanol, or with certain other water-miscible liquids, it is well known that a substantial change in the water content has only minimal effect on the overall conductivity of the combined liquids. The following Table I derived from direct conductivity measurements of ethanol and water solutions shows the minute changes in conductivity produced by changes in the water content:
TABLE I ______________________________________ Conductivity of Ethanol/Water Mixtures as a Function of Water Content % Water Conductivity (microsiemens/cm) ______________________________________ 0 0.82 1.25 0.83 2.50 0.85 3.75 0.88 6.25 0.91 ______________________________________
A five-fold increase in water content, from 1.25 to 6.25 percent, yields only a 0.08 microsiemens/cm change. This extremely small change has made the direct measurement of conductivity an impractical indicator of water content in the past. It should be noted that this deficiency is even more pronounced when the water and alcohol systems being measured are in the range of 99-plus percent alcohol, since a fraction of a percent change in the water content produces an almost negligible change in the overall conductivity of the system.
Therefore, it is an object of the present invention to provide a method and apparatus for making practical on-line determinations of water content by means of a conductivity measurement, in an accurate and timely manner.